C CLASP (pre-1800): The C Clasp was the first style of clasp used in jewellery. The earliest ones are just a rounded wire. Some other early styles include a more solid piece with a flattened top. In most cases, there is not much to hold the pin in place.
BACKUP PIN (1830-1850): used before safety clasps
PINS (1860-1889): Pin extending past the clasp could be early Victorian. Shorter pins are usually later.
SAFETY PIN (1880-1890s): attached with chain
EARLY SAFETY CLASP (1890-1910)
TUBE HINGE (until 1890s): the oldest type of hinge, the pin stem is attached to stationary end tubes on either side of the stem by a pin that goes through all three tubes.
STANDARD HINGE (1890s-present)
TUBE CLASP (1890-1940s), in Europe until 1950s
LOCKING C CLASP (1900+): Most brooches used C Clasps up until about 1900. Around this time, the locking C clasp was invented which used a locking mechanism to keep the pin from falling away from the C. Most brooches made today use some version of the locking C Clasp.
MODIFIED C SAFETY CLASP (1920s-present): new safety clasp
SOLDERING: Check for replacement soldering, which indicates faults and lesser value.
AIR BUBBLES: Air bubbles seen in stones is an immediate clue that the piece is glass.
BEZEL SET STONES: Fine Jewelry containing Diamonds or Precious Stones such as Rubies, Emeralds and Sapphires are either bezel-set, inset or set with prongs and will always have open-backs for maximum brightness.